As a precursor.to buying a chuck/chucks for an older (silver handwheel) Sherline lathe I bought recently, I had in mind to utilize 1 or more of these Sanou chucks I've accumulated. There actually isn't a backplate, threads are cut into the body of the chuck.
That's a challenge right there. Not "hard" in any way, it's just a LOT of variables to manage, and you're going to need to figure out which surface(s) actually register. It's probably quite simple, but it's probably not the threads. It'll also want to be a well fitted set of threads... That leads to boring the threads vs tapping them, and that would also require (maybe as a fringe benefit in this case), that you make up a "thread gauge" to match the spindle you're fitting the chuck to. Working "to the numbers" is not really ideal for a one time thread, and isn't really practical until you know your machine, your tooling, and have knocked out enough to know what the high and low tolerances are going to be for something produced that way.
Do you have "any" chuck for the Sherline lathe? If so, a loose chuck that DOES fit would be a great help, as you could make a male thread to fit that WELL, and that would in turn become the thread gauge for cutting the "new" chuck that doesn't currently fit.
Nose adapters aren't readily available, amd don't represent good methodology imo. Using a mt1 adapter isn't a stable option either (although someone suggested adding a drawbar so there's extra stability, alleviating the tendency of the adapter to spin in the spindle bore).
I kind of agree with that. Either would "work", but neither is ideal. Threading the chuck is best.
I've never cut a thread in my life, and although I really would like to learn, the best bet is to utilize a thread, I suppose. Thing is I'm a little lost with regards to thread nomenclature and designation.
You're not gonna take that all on in a day.....
But, if you've got a threading tool (one external, one internal), that will cut the required pitch... Short of "knowing what you're doing", we can help you out with getting that part going.
I have figured out that utilizing a bottom(ing) tap isn't a good idea. So I'm asking for recommendations.
Loads of stuff to know about taps, a lot of which comes with an understanding of the whole thread cutting thing. But here's some basic's to get you going for a simple project-
There are three "types" of taps that you need to know about.
A "taper tap" is just that. It's got a long taper in the beginning, so it starts easier (and truer if working by hand), and spreads the cutting across many, many cutting edges, so it drives easier. That'd be the first one you'd use if you were using three taps.
A "plug tap" is similar, but the taper only comes up a few threads. It varies, but let's say five or so. It's a lot more effort to turn it into a new hole, it doesn NOT have any desire to stand up straight when hand tapping. With effort and care however, this is the tap that can "do everything" kind of. It's the one that is found in kits, or blister packed at the hardware store where there are no choices in which type of tap you get.
A "bottom tap" is for tapping threads as far as is practically possible, and is typically reserved for tapping threads as close as possible to the bottom of a blind (or shouldered) hole. It has a taper only on the first couple of threads.
You will have to evaluate what the finished product would look like before threading. I doubt you need a taper tap, you will need a plug tap, and if the hole is going to have a "step", or diameter reduction at the bottom of the bored hole, you'll need to measure as you MAY (not necessarily, but you may) need a bottom tap as well.
There are two "cutting geometries" that you need to know about.
A "hand tap" has the flutes ground parallel to the center axis. These are the ones that come in a "standard" kit. These have to be "backed up" to break the chip every quarter turn to one full turn (depending on a LOT of things...). You can't (realistically) run them in a machine operation, although if you're hand feeding (manually turning a lathe chuck for example) they'll do fine.
A "spiral point tap" is technically a machine tap, however they are also excellent for hand use. They have a little angled grind on the lead in that sends the chips down the hole, instead of packing in the flutes. Often in hand operation (or manually turning a machine) these will still need to be backed up like a "hand tap".
And a "spiral flute tap". This is NOT what you want for this kind of job, however I bring it up because it should NOT be confused with the spiral point tap that I mentioned.
And tolerances... Yet another rabbit hole that goes deeper than you want to worry about
I'm just gonna say that H2 or H3 should make threads quite suitable for this.
For one hole? I don't think I'd even worry about coatings or materials. It's gotta work once. As long as it doesn't come from the farm supply store, it'll probably be durable enough.
I may only be converting 1 chuck, so I don't need something to last for decades. From what I've determined, there will be enough material left to cut 3/4"-16tpi threads after I machine off the M14 x 1 threads that are currently present.
Am I just better off using an mt1 adapter with a homemade drawbar?
Here's my suggestion-
Get some scraps (an inch bar would be a good compromise), bore some holes and turn some threads in 'em. Turn some of it down and make some male threads. See how you come out. See if you can get the precision fit that you'll need to get a chuck to run true. If you can (and it's a lot of stuff to manage on the first time out...), but if you can get good fits- Well, there you go. That's the ideal solution. (but do bug us again about fixturing the victim chuck). Or if the threading doesn't go well... Well, there you go.
Here's one tip for practicing- Truncating the threads...
When you're making the hole for boring a female thread, this is already included in the "tap drill size", which becomes the hole diameter. You want to aim for a 60 percent or slightly (but not much) higher thread engagement. So in this case the ideal "tap drill" (the size you'd bore to) would be about 0.701 inches. The 75 percent bore diameter would be 0.689 inches. Don't go smaller than that for any reason, shoot for the 0.701 number, close as you can without going over.
For the male threads on your "test pieces", the starting diameter is also not "nominal". It's a hair under that. So when you prep a blank to be externally threaded, it's rough outside diameter would be LESS than 3/4 inches by 10 percent of the thread pitch. The pitch of a 16tpi thread is 0.062, ten percent of that is 0.0062. So instead of a nominal 3/4 inch (0.750 inches), the starting "bar" would be 0.744 inches.
